MANCHESTER, England — It was that moment after the music ends. The pop star Ariana Grande had finished the encore of her “Dangerous Woman” concert, and the shrieks of teenagers and others had subsided. The stage show was over, the arena lights had gone up, and fans were clutching pink balloons that had dropped from the rafters — souvenirs from a special night.

Lisa Conway, 49, had secretly bought tickets a month earlier and booked a room at the nearby Park Inn Hotel. It was a surprise for her 14-year-old daughter, whose favorite artist is Ms. Grande. Mother, daughter, father and son came down from Glasgow, but the father and son skipped the show for a night on the town. The concert was a bonding trip, one of those markers of adolescence, and a small, tentative step into the adult world.

Then, with the arena still tingling with the exhilaration of the music, came the explosion.

“It was meant to be a dream, not a nightmare,” Ms. Conway said Tuesday morning while eating breakfast at the hotel, trembling as she struggled to contain tears. “There were children, blood, shoes, splattered all over the floor.”

She added: “How can I explain any of this to a 14-year-old? She hasn’t said a word since she woke up from two hours’ sleep.”

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Children, teenagers and adults tell of seeing bodies on the ground and of scrambling to escape the Manchester Arena when a bomb exploded after an Ariana Grande performance.CreditCreditZach Bruce, via Press Association

The explosion at Manchester Arena on Monday night — carried out, the police said, by a man who died in the blast — was the worst terrorist attack in Britain since 2005, with at least 22 people dead and dozens more injured. The man was later identified as Salman Abedi, 22, a Briton whose family emigrated from Libya. Although the Islamic State claimed responsibility for the attack, the police said they were still investigating whether Mr. Abedi, who lived in Manchester, had acted alone or as part of a larger plot.

Terrorism has its own language and symbolism, whether it is an assault against a satirical newspaper in Paris or the bombing of a luxury hotel in Mali. The violence is intended to stoke fear and to deliver a message. And it was the message of the Manchester blast that was so chilling: the slaughter of teenagers, the anxiety of parents who had been waiting to take their children home, the frantic search for loved ones amid chaos and sirens.

Already, questions are emerging about whether the arena was properly secured. Investigators said the explosion occurred in a foyer, though it was not yet clear whether this referred to a public space between the arena and the nearby Manchester Victoria rail station or to an area inside the security perimeter of the arena.

Concert organizers said security had been tight, with close checks of ticket holders. But some concertgoers described a much looser environment.

“Our bags were searched as we were going into the arena, but only quickly,” said Caitlin McCoy, 23. “I had a secret pouch in my purse — it only had makeup in it — but they did not look into it. They seemed to be more concerned about finding alcohol than anything else.”

The concert began around 7:35 p.m., and after two warm-up acts, Ms. Grande took the stage. Once a child star on the television network Nickelodeon, Ms. Grande, 23, is known for her strong voice and, like other child stars, has sought to evolve with her fans as they grow up together.

Her debut album in 2013, “Yours Truly,” reached No. 1 on the Billboard 200 in the United States, and her May 2016 “Dangerous Woman” album signaled her progression into more mature themes. The title song is about wanting to be a “bad girl” as a form of empowerment — to be strong and fearless — and the album quickly went to the top of the Official Albums Chart in Britain.

As Ms. Grande finished her encore and left the stage around 10:30 p.m., the lights went up and people began streaming toward the exits. Then came what one person described as an “almighty explosion.” Some witnesses recalled smelling sulfur. A gush of air whooshed through the arena. For an instant, no one knew what had happened. Some people wondered if some of the pink balloons had burst.

At first, Ms. McCoy told herself it could not be a bomb. The pall of white smoke made her think it was a firework released after the final song, a last pyrotechnic from the spectacular stage show. “I thought a bomb would look different, sound different,” she said. “I don’t know why.”

When Ms. McCoy and her friend Amy Hedley had taken their seats in the upper tier of the arena, they had remarked to each other on the youthfulness of the crowd. “It was mostly teenage girls,” she said. “The average age would have been about 14.”

The power of the blast shook the arena, and many people started screaming and running. Outside the hall, parents had been waiting to pick up their children. Diane Burnett, from Edinburgh, was looking for her 17-year-old son at the time of the explosion. “It was a loud bang, and then silence, and then loads of girls screaming,” she said. “You didn’t know what it was, whether it was a train crashing.”

Another parent, Kevin Pickford, rushed into the main entrance to search for his two daughters. “There was an announcement, asking people to leave slowly and calmly,” he recalled. But panic was overtaking the calm.

“Everyone was crying and screaming,” said Sophie Tedd, 25, who attended the concert with her friend Jessica Holmes. “Nobody knew which way to go.”

One older woman in a wheelchair was trapped in the tumult as another spectator shouted at the crowds to let her through.

Molly Cronin, 18, described a mini-stampede of panicked fans. “Kids were getting crushed, and we were trying to help them,” she said, noting that the timing of the blast seemed “deliberate.”

Outside, police officers were quickly on hand, trying to direct people to safety. “They were telling us to run, to keep running, away from Victoria station,” Ms. Tedd said. Fleets of wailing ambulances were pulling up to the scene.

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Police officers patrolling in Manchester, England, on Tuesday, the day after the deadly attack at an Ariana Grande concert.CreditChristopher Furlong/Getty Images

Few people, though, knew where to go. A police cordon quickly took shape. Local hotels opened their doors, offering sanctuary to the stranded.

Inside the arena, Louise Reid, 48, was leading her 15-year-old daughter, Patty, toward the exit when the crush of people overwhelmed them. “I turned around and then felt like hundreds of people were falling on me,” she said, describing how the power of the crowd swept her away from her daughter. “I couldn’t turn back. I felt so helpless.”

When she reached the area where the blast had occurred, Ms. Reid was horrified, she said. Parents and others were frantically looking at bodies, trying to identify loved ones, until security guards ushered everyone out.

Ms. Reid was one of the frantic parents, searching for her daughter. “But Patty never came out, and I refused to leave until I was forced away by the paramedics and the police,” she said. “I didn’t know what to do, where to go. I didn’t have a phone. I just kept screaming for Patty.”

Three hours passed before the police found Patty at a nearby hotel. “Three hours of not knowing whether my daughter was dead or alive,” Ms. Reid said, with tears streaming down her cheeks as she sat in the home of a resident near the arena.

“How do we go back to normal after this?” she asked. “The tragedy shows that this can happen anywhere, at any time.”

Thomas Furse contributed reporting from London.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: A Joyful Night, Then a Loud Bang and Blood ‘Splattered All Over’. Order Reprints | Today’s Paper | Subscribe